1. Field of Invention
This invention relates in general to methods of testing surgical implants for the eye. More particularly, the invention relates to methods for the in vitro testing of one-way, pressure gradient limiting valved glaucoma drainage implants prior to the surgical placement of such implants into the tissue of a mammal during the surgical treatment of glaucoma.
2. Summary of the Prior Art
Glaucoma is an eye condition in which, due to various causes, the intra-ocular pressure (that is the pressure of the aqueous humor in the eye) rises. This rise in intra-ocular pressure tends to make the eyeball hard. Further, in high-tension glaucoma, the rise in intra-ocular pressure tends to adversely affect vision, and may cause partial or total loss of sight.
Various methods for surgically treating glaucoma have been developed over the years. Significant among these methods is the surgical implantation of drainage devices which utilize drainage tubes to maintain the integrity of openings formed in diseased eyes for the flow of aqueous humor. In such implant devices, a drainage tube typically provides a passageway designed to extend from the anterior chamber of the eye to a drainage body sutured to the sclera of the eye. The purpose of the drainage body is to increase the available drainage area so as to ensure that aqueous humor drains away from the eye and is absorbed by the body at a sufficiently high, but controlled, rate.
To this end, non-valved aqueous humor drainage implant devices, such as the well knows Schocket Tube or Molteno Glaucoma Drainage Implant, have been utilized. These devices rely upon the back pressure created by a so-called "bleb" of aqueous humor which forms over the drainage body prior to absorption by the body to control the rate of flow of aqueous humor away from the anterior chamber of the eye. These devices, however, have been found to be not totally satisfactory. This is because the flow rate of aqueous humor away from the anterior chamber of the eye is not controlled, at least initially. This tends to result in at least an initial overdrainage of aqueous humor from the anterior chamber of the eye immediately following implantation of the device. Such overdrainage can cause the eye to flatten undesirably, and can also lead to other complications.
For example, a subchoroidal hemorrhage may develop during glaucoma drainage or cataract surgery. As used herein a "subchoroidal hemorrhage" refers to bleeding into a potential space between the choroid (the highly vascular and pigmented layer of tissue adjacent to the retina) and the sclera. Subchoroidal hemorrhages are more serious events than serous, non-bloody, subchoroidal effusions which may represent fluid in and adjacent to swollen choroidal tissue. The latter fluid can resolve itself without scarring or disorganization of adjacent tissue, and without loss of visual acuity or even all vision. Subchoroidal hemorrhages underlying the macula (the area of the retina used for reading quality vision), however, commonly cause some permanent loss of visual acuity and are highly undesirable. This is particularly the case because they are followed by scarring and disorganization of adjacent tissues including adverse changes in light-sensitive cells in the adjacent tissue of the retina, and can result in the loss of all light perception.
To avoid the foregoing problems, numerous implant devices have been developed which include drainage tubes, drainage bodies and means such as one-way pressure gradient limiting valves to control the rate of aqueous humor flow from the eye. Significant among the latter type of devices is the so-called "Joseph device" which is described in detail in U.S. Pat. No. 4,604,087 issued Aug. 5, 1986 to the present inventor. The disclosure of that patent is hereby incorporated by reference into this specification.
In the normal eye of a human being, the pressure of the aqueous humor in the anterior chamber is, on average, typically between about 14 mm and about 16 mm of mercury. Further, it has been determined that a successful glaucoma drainage device is one that ensures that the intra-ocular aqueous humor pressure in the eye remains below 18 mm of mercury for at least six months following its surgical implantation. Hence, the valve opening pressure in the Joseph device is preferably between about 4 mm and about 20 mm of mercury as determined by in vitro testing prior to implantation.
A problem of significance to surgeons, however, remains. This problem relates to the in vitro testing of one-way, pressure gradient limiting valved glaucoma drainage implants prior to their surgical implantation in a patient in association with an eye. Specifically, there is a desire in the art to be able to ensure that a pressure gradient limiting valved glaucoma drainage implant device is both functional and appropriately calibrated prior to its implantation into a patient. Heretofore, a comparison of in vivo and in vitro testing results indicates that current in vitro testing practices do not satisfactorily predict how the device will function in vivo.
The reasons for this are not entirely clear. It is believed, however, that the primary causes of the lack of correspondence of in vitro and in vivo testing of pressure gradient limiting valved glaucoma drainage devices resides in the facts that (1) current in vitro ophthalmic testing methods are not dynamic, and (2) current ophthalmic testers have failed to fully comprehend the environmental, material and geometrical factors inherent in the devices tested and the test procedures adopted.
The eye normally has pressure fluctuatingly applied to it by the effects of heartbeat (pulse), breathing, crying, temperature, level of motor activity, variations in aqueous humor secretion over time, and changes in posture or bodily orientation, among others. Further, the in vivo environment consists of wet tissue. Nevertheless, current ophthalmic testing procedures are typically conducted in a gaseous environment (i.e., air), and apply only a gradually varying absolute pressure gradient to the valve of the device.
It has been found that such testing can result in differences in measurements of the opening pressure of the valve of the device being tested taken within an hour of each other which exceed 10-20 mm Hg. This is an unacceptable repeatability of results. In fact, the Apr. 20, 1996 work group meeting of the American National Standards Institute Z-80 Committee on Glaucoma Drainage Implants has gone so far as to indicate that "Non-physiologic flow rate studies and studies done in air do not contribute useful information to the user (of one-way pressure gradient limiting valved glaucoma drainage implants)" (parenthetical added).
Accordingly, it will be understood that current ophthalmic testing does not approximate the environment into which the implant is to be placed for operation. It also will be understood that due to the viscosity and elasticity of silicone rubber and the presence of van der Waals forces between the clean, smoothly cut opposed surfaces of slit valves formed therein, the slit valve flaps tend to stick together unless subjected to physiologic flow of aqueous humor or cerebrospinal fluid, constantly irrigated or otherwise treated. Further, it will be understood that testing in air creates an air/liquid interface at the slit valve exit. This interface introduces a surface tension effect which must be overcome in opening the slit valve thereby unacceptably detracting from the meaningfulness of the test results generated.
Testing in a liquid environment (i.e., with the slit valve located in a water bath) has been conducted previously. This reduces or eliminates the surface tension effect when water is used as the fluid applied to the valve under pressure to determine its opening characteristics. It, however, does not fully deal with this problem when other substances more closely allied with the composition of aqueous humor are utilized as the pressure applying liquid.
Studies of hydrocephalus shunt valves and heart valves have heretofore noted the importance of pulsate flow, temperature and time-in-service on valve performance. However, whatever potential relevance these studies may have in the very specialized ophthalmic context either has gone unrecognized in the art, or has been discounted because hydrocephalus shunt valves are required to function at much higher flow rates and with much higher pressure fluctuations than are expected in eyes.